This invention relates to milling machines and cutting tools usable in milling machines. In particular, the cutting tool of this invention has a helical shaped cutting surface for machining the inside diameter of a turbine rotor.
This invention is related to the subject matter of commonly assigned copending U.S. patent application Ser. No. 08/904,155 (attorney docket number T196012) filed on Jul. 31, 1997, now U.S. Pat. No. 5,844,191, entitled "Method of and System For Manufacturing A Helical Cutter," which is hereby incorporated by reference.
Milling or boring machines may employ a variety of cutting tools. A cutting tool may be of the type that has a longitudinal axis with teeth disposed around the circumference of the longitudinal axis. These cutting tools can be inserted into a hole and rotated about their longitudinal axis to bore a larger hole to a specified shape, such as a larger diameter. Tools of this type are well known and may be employed in manufacturing tubular shaped structures, such as a turbine rotor.
A cutting tool may be classified as either a standard cutting tool or a form cutting tool. A standard cutting tool is prefabricated to certain dimensions and can typically be purchased by specifying a stock number or the like. In contrast, form cutters are those that are designed to cut a work piece to a specific shape. Form cutters are typically manufactured to specific dimensions as specified by the purchaser. As those of skill in the art will appreciate, tools used in the manufacturing of turbine rotors are typically form cutters.
During cutting operations chips are created. These chips vary in size and shape depending on the type of material being cut, the material doing the cutting and in large part on the geometry of the cutting tool. The size and shape of the chips, and consequently the geometry of the cutting tool is important because it affects the speed of cutting, tool wear, surface finish, safety of a cutting operation, machining tolerances and other characteristics of the cutting process. For instance, continuous chips generally produce a good surface finish but may present a safety concern for the tool operator. In comparison, segmented chips may cause a severe distortion of the metal in the area adjacent to the tool and cracking of the work piece. One way to control the type of chip generated is to select the proper geometry of a cutting tool. Thus, the geometry of a cutting tool is a critical feature in controlling the cutting process.
The tool geometry may also affect the force or torque required to cut a work piece. Typically, the tool face of a cutting tool, the surface against which the chips bear, is inclined to either increase or decrease the keenness or bluntness of the cutting edge. Conventionally, the inclination of the cutting face is referred to as the rake angle. Since the tool face may be inclined in more than one direction, a cutting surface may have more than one rake angle. Rake angles can be either positive or negative. A rake angle is positive if the cutting edge leads the surface of the tooth face with respect to a work piece and negative if it lags behind the tooth face. Positive rake angles tend to reduce the requisite cutting force or torque and direct chip flow away from the work piece. In contrast, negative rake angles generally increase the required cutting force, but provide greater strength at the cutting edge.
Conventionally, cutting tools used to manufacture turbine rotors have a plurality of teeth disposed axially and circumferentially along the cutting tool. Each tooth is disposed along a spline extending axially from the front of the tool to the back of the tool. Each spline extends parallel to the longitudinal axis of the tool. The cutting face of each tooth along the spline is substantially parallel to the cutting face of the other teeth disposed along that spline. Additionally, the teeth are disposed at a rake angle of zero in both the axial and radial directions. The tolerance of cuts made with a tool of this geometry is limited. Furthermore, the efficiency of the cutting process and the cutting speed are also limited with this type of tool. Moreover, the force or torque required to cut with a tool of this geometry is relatively substantial.
Because of the limitations of conventional cutting tools, an improved cutting tool used to manufacture turbine rotors is needed. Conventional methods and systems of manufacturing cutting tools have prevented the development of such an improved cutting tool. In particular, cutting tools of this type were conventionally manufactured with a milling machine or similar cutting machine. Because of the limitations inherent in milling processes, the geometry of cutting tools manufactured with a milling process is limited.